Atrial fibrillation is a condition in the heart in which abnormal electrical signals are generated in the endocardial tissue to cause irregular beating of the heart. One method used to treat atrial fibrillation involves creating several long (i.e. approximately 2-10 cm) lesions on the endocardium within the atria. These lesions are intended to stop the irregular beating of the heart by creating barriers between regions of the atria. These barriers halt the passage through the heart of the abnormal currents generated by the endocardium. This procedure is commonly referred to as the "maze procedure" because it creates a maze of lesions design to block the passage of abnormal currents through the heart.
Existing procedures for forming such linear lesions include the highly invasive technique of opening the patient's chest and heart and forming linear incisions inside the atria. Naturally, the highly invasive nature of this procedure makes it a particularly high risk to the patient and necessitates extraordinarily long recovery time.
Other attempts have been made to form the linear lesions using ablation catheters fed into the heart via the patient's vessels (i.e., the arteries or veins). For example, one such procedure involves inserting into the atria a 7 French catheter having an ablation tip. Radio frequency (RF) energy is supplied to the tip as the tip is dragged across the endocardium, thereby burning linear lesions into the endocardium.
While often successful for forming linear lesions, the ablation tip of the catheter can sometimes lift off of the surface of the endocardium as it is dragged across the endocardium, creating one or more breaks in the lesion. Such breaks minimize the success of the ablation procedure by leaving a path through which current may travel during atrial fibrillation episodes.
Another type of existing RF linear lesion catheter has a plurality of spaced ring electrodes encircling the distal end of the catheter. Although ablation using such catheters has been successful, a large spacing (i.e. on the order of greater than 4 mm) must be provided between the ring electrodes to give the catheter adequate flexibility to move through the vessels and the heart. Unfortunately, however, leaving large spaces between the electrodes may prevent RF energy from being focussed at certain points along the catheter and may thereby produce regions in the linear lesion which are not sufficiently necrosed to prevent the passage of current during atrial fibrillation episodes. Additionally, blood may coagulate on the surface of the metal ring electrodes, which can prevent RF energy from reaching the endocardial tissue. Coagulum on the surface of the electrodes may present a safety hazard in that it may eventually fall off the electrode and into the patient's bloodstream, causing an embolic event.
Procedures and devices for forming linear lesions within the atria are therefore desired which will block the passage of current through the heart during atrial fibrillation episodes, as with the surgical incision procedure, but which utilize the less-invasive technique of a percutaneous catheter. Further desirable is a linear lesion catheter having the flexibility and maneuverability of the electrode tipped catheter but which generates a continuous lesion on the endocardium.
It is further desirable to improve the continuity and thus the effectiveness of linear lesions formed using ablation catheters by providing means by which a linear lesion catheter may be held securely against endocardial tissue during ablation, and by which electrical energy may be focussed from the ablation electrodes onto the endocardium.
A problem in cardiac ablation is the phenomenon of "popping". Popping refers to explosions in the body's tissue which are observed during RF ablation. Popping is generally undesirable because it causes irregularities, such as small tears, in the ablated tissue, and therefore it becomes more difficult to precisely control which tissue is ablated. Also, the force of the popping explosions can cause and disperse coagulum. The exact mechanism by which popping occurs has not been not fully explained.